374 Rev. Mr, Powell's experiments relative to 
through glass, it perhaps would not necessarily follow, that it 
might not, under peculiar circumstances, have a power of doing 
so communicated to it. Though on the other hand it must 
be confessed that, in the present case, some difficulty would 
attend such a supposition. 
It certainly would not be easy to conceive such a property 
to be communicated to the heat by the mere act of being con- 
ducted through the first glass. Again ; a new property of 
heat is thus introduced which, it must be conceded, is not 
absolutely and exclusively established. 
It appeared to me therefore a point of some interest to ex- 
amine, in the case of non-luminous heat — in the first place, 
the accuracy of the fact ; and secondly, if verified, whether 
there might not be circumstances observable in the condi- 
tions of the experiment by which it might be accounted for, 
without the necessity of supposing any peculiar property of 
heat, or a direct transmission even through the second glass. 
(4.) My apparatus in following up this enquiry was similar 
to that described by M. De La Roche, and consisted of two 
tin reflectors. In one focus the bulb of a thermometer coated 
with Indian ink, and in the other an iron ball two inches 
diameter, which was heated to redness, and then cooled till it 
ceased to be visibly red in the dark, at which point it was 
placed on its stand, and a thick screen withdrawn. The in- 
dications were observed, first for the direct effect ; secondly, 
with one glass screen interposed ; and thirdly, with two. In 
this way I tried various experiments ; in some using only 
one reflector ; in others both ; in some employing a mercu- 
rial, in others an air thermometer. I conceive it unnecessary 
to bring forward all the various experiments I tried, and 
